Tire Rolling Resistance (“RR”) tests have been performed for many years, but especially since the energy crisis of 1973. Referring to the prior art drawing of FIG. 1, such tests are commonly performed on a roadwheel tester 15 by loading a tire 110 against the cylindrical surface of a roadwheel 30 that has a diameter greater than 48 inches, usually of 67.23 inches diameter. RR tests on a round roadwheel have been found to correlate well with vehicle coastdown tests as long as the roadwheel diameter exceeds 48 inches. Roadwheel tests became the norm for measuring tire RR in the 1980's with the Society of Automotive Engineers publication of its Recommended Practice J1269 and later J2452. While road wheel tests give results that correlate with flat footprint tests, the result is a higher measured RR than a flat footprint would generate due to the round (cylindrical) roadwheel surface which causes a reverse curvature of the tire 110 in the area of its footprint 33, which is not representative of actual tire service.
Therefore tire and vehicle engineers have always been interested in performing RR testing by loading a test tire 110 against a flat surface test machine 35, for example the Flat-Trac® tire test machine illustrated in FIG. 2A, which is sold by MTS Systems Corporation of Eden Prairie, Minn., USA. Referring to the FIG. 2B schematic view of a prior art flat surface test machine 35, a test tire 110 is loaded against a flat section of a running belt 37 with a loading force of, for example, 1,000 pounds. A flat support surface 39 under the belt 37 counters the tire loading force to keep the belt flat. To control frictional drag, water 41 is forced between the belt 37 and the support surface 39 to provide a hydrodynamic bearing.
There are several problems inherent in rolling resistance tests performed on existing flat surface test machines 35. A major problem is due to an unknown road surface inclination angle that is created by a water wedge of the hydrodynamic bearing supporting the flat track against which the tire is loaded. In effect the hydrodynamic bearing presents the road surface to the tire at a small, but unknown, inclination angle that causes uncertainty in the RR test results. A paper presented by Lloyd of the General Motors Proving Ground (Stephen E. Lloyd; “Development of a Flat Surface Rolling Resistance Test Facility”; SAE Paper 780636; Jun. 5, 1978) described a flat surface test machine especially constructed for rolling resistance tests, but which was later dismantled due to the unknown footprint inclination angle that influenced the measurements by an unknown amount.
Another problem is that the prior art flat surface testers 35 are not only expensive to purchase, but the cost of running RR tests on them is generally too high for the test volume required. Therefore the MTS Systems Corporation is offering a variant flat surface tire tester 45 as shown schematically in FIG. 3. Such a test machine 45 could be made for less cost than the full force and moment flat surface tire tester 35. Furthermore, the variant tester 45 economizes by testing two tires 110a, 110b (collectively referred to as tires 110) simultaneously. However, the measurement errors caused by the water wedge 41 still need to be addressed.